Welding of metal products by using inductive radiofrequency currents is already known from the prior art. The radiofrequency welding is carried out contactlessly, and in particular makes it possible to produce mass products in continuous throughput methods. In inductive radiofrequency welding, an induction conductor carrying radiofrequency currents induces oppositely directed radiofrequency currents in the metal product to be welded, and these lead to strong heating of the metal product at the corresponding positions carrying radiofrequency current. In general, closed current loops are always induced, with essentially two effects playing a role. These are on the one hand the “proximity effect” which means that the induced current densities are greatest in the immediate vicinity of the induction conductors. Furthermore, owing to the “skin effect”, the current transport of the induced radiofrequency currents takes place not inside the metal product, as for instance with a direct current, but essentially (i.e. substantially) on its surface. The induced radiofrequency currents travel in closed paths, which can be influenced by the arrangement of the induction conductors. In general, as a rule, the induction conductor is arranged so that the return current paths are concentrated on edge regions of the metal products owing to the “skin effect”. Then the edge regions through which current flows are correspondingly strongly heated. The edge regions heated in this way are brought in contact with one another and welded together to form a weld seam.
A corresponding method is known from US patent specification U.S. Pat. No. 4,197,441. In the known method, an induction conductor which is adapted to the weld seam to be produced, is used to correspondingly heat the marginal regions of the metal products to be welded. A problem with the known method is that the heating of the marginal regions takes place very differently for metal products with different wall thicknesses or consisting of materials having different electrical resistances, so that stable process management cannot be ensured when corresponding metal products are being welded. However, the welding of metal products with different wall thicknesses or consisting of materials having different electrical resistances is necessary for the production of semi-finished products, for example tailored products. Tailored products generally have different wall thicknesses and/or material properties or substances, which are adapted to loading. Differently thick metal products or metal products consisting of different materials are often welded together in order to produce corresponding tailored products. Such metal products may for example be flat metal products, known as “tailored blanks”, cut sheet metal shaped to form hollow sections or tubes, known as “tailored tubes”, or cut sheet metal shaped in strips, known as “tailored strips”. A large application field of tailored products is automotive manufacturing, since tailored products have an optimal ratio between maximum loading and weight, and in this regard can be adapted very well to the respective application task.